In the past few years, fiber optic-based strain gages have gained acceptance in the market as an alternative to conventional electronic gages which are based, for example, on changes in electrical resistance, inductance or capacitance. In many applications, such as civil structure monitoring, down-hole oil and gas applications, as well as marine, and aerospace applications, fiber optic gages offer several advantages over conventional gages. Unlike electronic gages, fiber-based gages are not sensitive to electromagnetic interference and are well suited for use in electrically-noisy environments. Fiber-based gages are also readily multiplexed, allowing many gages to operate on a single fiber over long distances. Fiber-based gages can be made very small and lightweight for use in confined spaces. Fiber-based gages can also be made to withstand high temperature and corrosive environments.
A wide variety of optical gages are available that utilize various optical properties to measure a physical property of interest. These gages are designed to respond to the physical property to be measured by a change in amplitude, phase, polarization state, or other optical property of light transmitted through the fiber. Depending on the gage design, one or more of these optical properties can be monitored by an interrogation unit and converted to the physical property of interest.
The current invention relates to optical gages designed to measure strain at the surface of a test specimen. The gages of this invention are designed to be installed and used in a manner similar to conventional electronic foil strain gages, but to have the advantages of an all-optical gage. Gages of this invention are constructed to allow surface strain on the test specimen to be transferred to a length of optical fiber containing a Bragg grating. As strain is applied to the fiber, the optical spectrum center wavelength reflected by the Bragg grating shifts in wavelength. This shift in wavelength can be converted directly to units of strain.
In this type of gage, the fiber is often mounted to a carrier for ease of mounting onto the test specimen and ease of handling. The gage carrier can be made of a plastic film or thin metal shim. Plastic films work well in applications where the gage is applied to the test specimen with epoxy or other adhesives. Metal carriers are commonly used in applications where the gage is spot welded to the test specimen. Strain gages mounted on metal carriers are versatile because they can be installed quickly on various test specimens. Although the use of a carrier for such gages significantly improves handling and mounting, its use can also degrade gage performance. The gage carrier is positioned between the test specimen and the sensing element, requiring that strain from the test specimen is passed through the carrier to the fiber. This can result in a loss of sensitivity. Further, the gage carrier must be carefully mounted, so that strain is passed through the carrier fully and uniformly without deforming the carrier. Distortion of the carrier other than by pure linear strain can result in loss of accuracy of measurement because the sensing element is not exposed to the true strain present in the test specimen. Additionally, the stiffness of the carrier material itself can influence the strain measurement by reinforcing the stiffness of the test specimen. This occurs in electronic foil strain gages as well as fiber based optical gages.
U.S. Pat. No. 6,834,552 relates to a fiber-optical strain-gauge in which strain conveyed from a test specimen changes the distance between two fiber ends which in turn results in a change in transmission of light between the two fiber ends. The optical fiber ends are held aligned in a carrier. The carrier construction is described as providing a strain-dependent, relative movement of the optical fiber ends in a direction substantially perpendicular to the axial direction of the optical fibers so that a change in transmission of light between the fiber ends is a measure of the strain applied to the carrier from the test specimen. A carrier is described as having two fiber holding elements in which the two fiber ends are held aligned with a gap established between the ends, two fixation zones for attaching to the test specimen, and two double-hinge zones which convert axial relative movement of the fixation zones into oppositely directed vertical movements of the holding elements and the fiber ends held in the holding elements. Additionally, a carrier is described in which the two double-hinge zones convert axial relative movements of the fixation zones into oppositely horizontal movements of the holding elements.
U.S. Pat. No. 7,068,869 relates to a passive athermal fiber Bragg grating (FBG) strain gage in which the carrier or package of the fiber Bragg grating is constructed to cancel temperature sensitivity of the gage. The strain gage is described as having a frame and pointer with a fiber-to-frame engagement means and a fiber-to-pointer engagement means, and means for attaching the frame to a structure to be monitored. The pointer is an element having threads which is threaded through an aperture in the frame so that the length of the pointer extending into the frame can be adjusted which in turn allows the length of the fiber segment between attachment points on the frame to be adjusted. The frame is described as having “flexures” which “allow strain at minimum stress to be applied to the FBG.”
U.S. Pat. No. 7,134,219 relates to a fiber optic gap gauge which employs a Fabry-Perot interferometer (FPI) strain sensor. The gauge is described as having a base with an elongated tension bar, a pair of end connectors adjacent opposite ends of the tension bar and at least one spring connecting each end of the tension bar to a corresponding end connector. The tension bar is provided with a hollow cavity and slotted grooves so that the strain sensor can be introduced to measure strain in the base and particularly in the hollow cavity of the base. A bow spring having a pair of ends is described as being connected through its ends to corresponding end connectors of the base with middle section of the bow spring bowed away from the base to define a gap between the middle of the bow spring and the base. Application of a force to the middle of the bow spring decreases the gap and exerts a tensile force axially on the base. Strain is measured using the (FPI) strain sensor and strain is indicative of the change in the gap. The strain sensor does not contain a fiber Bragg grating and the optical fiber conveys light between the strain sensor and an external Fabry-Perot cavity where light is modulated.
Published U.S. patent application 2007/0193362, which is commonly owned with the present application, relates to a gage carrier design for use with fiber optic strain sensors comprising one or more FBGs which provides the benefits of a carrier for ease of handling and mounting without degrading gage performance. The carrier is made of any appropriate material including metal, ceramic or plastic. The gage carrier is selectively compliant along the axis of strain measurement. The carrier provides very little resistance to strain along the axis of measurement; therefore, the test specimen does not have to work against the stiffness of the metal gage carrier. More specifically, the gage carrier comprises a support bar extending along a longitudinal axis of the carrier, with first and second mounting surfaces positioned and rigidly attached at an end of the support bar for mounting the strain gage onto a surface of a test specimen. The gage carrier also has fastening elements for securing an optical fiber parallel to the longitudinal axis of the carrier. At least a portion of the support bar is elastic with respect to expansion, compression or both along the longitudinal axis of the carrier.
While a number of strain gage configurations are available in the art there remains a need for fiber optic strain gages which are easy to handle and rapidly installed without any significant loss of performance.